Without using my imagination (who has imagination when they are exhausted all the time), I think it is the evil marketing machine. After you get all the little artifacts of the drug trade... ahem! I mean air traffic business, then you have to buy the big bucks airport to go with it. YOU HAVE TO BUY IT otherwise you have just a mishmash of pieces that don't make sense... so BUY the pricey big airport!

Holy cow! I thought I was kidding then I did a Lego search:http://shop.lego.com/ByTheme/Product.aspx?p=K10159&cn=92